1. Field of the Invention
This invention relates to solenoid valves, and more particularly to an arrangement of solenoid valves in instrumentation and process control systems of a plant.
2. Background Information
Modern process or manufacturing plants contain innumerable operating components. These components are tied together to form systems controlled by instrumentation and control systems containing sensors and controllers. The instrumentation and control systems on such plants not only serve to control the functions of the various components in order to achieve the desired process conditions, but they also provide the facility to safely modify or discontinue the operation of all or a portion of the plant's systems in order to avoid an unsafe situation or condition.
Safety systems or configurations require routine testing in order to verify that they continue to properly perform the functions for which they were intended. From an operational and economic point of view, they also should not modify or discontinue the operation of the plant system unnecessarily. One of the means by which such safety systems function is by the securing or diverting of the supply of a certain process fluid or the supply of motive power to a plant system or component of a plant system. One of the means by which these safety functions may be accomplished is through the use of solenoid operated valves.
In operation, the solenoid valves of such systems serve to isolate and/or vent off the fluid or pneumatic source from the system when the solenoid valve changes state or position (e.g. when the valve is de-energized by switches or process monitoring sensors coupled thereto). The plant system and any system controlled thereby is then placed in a configuration designated for safety.
In many cases, the operation of individual solenoid valves may not be tested without actually tripping the system and undesirably modifying or discontinuing the operation of the plant system. Moreover, various configurations available for actuating safety shutoff valves generally require a trade-off between competing characteristics of safety and spurious trip rate.
For example, a "1 out of 2" voting solenoid valve arrangement generates a process modification (i.e. shutdown) when at least one of the two solenoids changes state. Such a configuration provides relatively high safety, with an associated relatively high spurious trip rate (i.e., a single faulty valve or sensor may generate a false trip). A "2 out of 2" voting solenoid valve arrangement requires actuation of two solenoid valves in order to trigger a modification of the plant system. This configuration has a relatively low spurious trip rate, since both solenoids must fail or otherwise change state to generate a spurious trip. However, the solenoid valves of this arrangement must be tested frequently to insure safety, since failure of only a single solenoid valve may effectively prevent the system from shutting down, etc.
Quad voting solenoids provide a balance of safety and reliability, however the quad configuration utilizes four solenoids in a relatively complex arrangement that tends to be difficult to install, test, and maintain.
Thus, there exists a need for a voting solenoid configuration which combines the features of relatively high safety with relatively low spurious trip rate and does not require the plant system to be shut down during testing.